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APPENDIX D

Assessment of Ship Impact Frequencies


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WHITE ROSEDEVELOPMENT APPLICATION

APPENDIX D

ASSESSMENT OF SHIP IMPACT FREQUENCIES

SUBMITTED BY:

HUSKY OIL OPERATIONS LIMITED AS OPERATOR SUITE 801, SCOTIA CENTRE

235 WATER STREETST. JOHNS, NF, A1C 1B6

TEL: (709) 724-3900FAX: (709) 724-3915

July 2000


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White Rose DA Volume 5 Part Two (Concept Safety Assessment) July 31, 2000 Page D-i

TABLE OF CONTENTS

Page No.

1 INTRODUCTION ..............................................................................................................................1

1.1 Authorized Vessels........................................................................................................................11.2 Passing Vessels..............................................................................................................................8

2 REFERENCES..................................................................................................................................12

LIST OF FIGURES

Page No.

Figure 1.1-1 Probability Distribution of Impact Energy for Supply/Standby Vessels ....................... 4

Figure 1.1-2 Probability Distribution of Impact Energy for Shuttle Tanker (Full) ............................ 4Figure 1.1-3 Probability Distribution of Impact Energy for Shuttle Tanker (Empty) ........................ 5Figure 1.1-4 Fault Tree to Estimate Frequency of Collisions by Authorized Vessels (FPSO).......... 6Figure 1.1-5 Fault Tree to Estimate Frequency of Collisions by Authorized Vessels

(Semi-submersible) ........................................................................................................ 7Figure 1.1-6 Fault Tree to Estimate Frequency of Collisions by Authorized Vessels (FSU)............. 8Figure 1.2-1 Fault Tree to Estimate Frequency of Collisions by Passing Vessels ............................. 11

LIST OF TABLES

Page No.

Table 1.1-1 Vessel Displacement ...................................................................................................... 2Table 1.1-2 Maneuvering Collision Mean Speed and Percentage of Incidents................................. 3Table 1.2-1 Sample Vessel Data........................................................................................................ 9


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White Rose DA Volume 5 Part Two (Concept Safety Assessment) July 31, 2000 Page D-1

1 INTRODUCTION

This Appendix presents the Input Initiating Frequencies (IIFs) for ship-installation collision risks due toauthorized and passing vessels, for the White Rose project, using Fault Tree Analysis. These IIFs will

be used as inputs to the event trees in the QRA Risk Profile model, in order to estimate risk levels.

1.1 Authorized Vessels

Any offshore installation must be supported by various vessels, providing a variety of specific services.The close proximity of shuttle tankers, supply/standby vessels, and other specialized ocean crafts (e.g.,diving operations vessel) are essential to any installation. Therefore, the approach in determining therisks due to authorized vessel collision is similar for all installations, regardless of location. However,the categories of authorized vessels that service a facility will depend on the type of installation utilized.For example, a ship-shaped floating, production, storage and offloading (FPSO) vessel will typically beserviced by supply/standby vessels and shuttle tankers because hydrocarbon production, storage, and

offloading operations are carried out at the same location, whereas a semi-submersible is normallyserviced by supply/standby vessels only. A semi-submersible is not typically used for storage. TheWhite Rose semi-submersible will be accompanied in the field by a floating storage unit (FSU), to storethe crude as it is produced and carry out shuttle tanker offloading operations. Therefore, there is nodirect need for shuttle tankers to venture within close proximity of the semi-submersible installation.For the semi-submersible option, there will still be a risk of shuttle tanker/FSU collision, though theFSU has a much smaller manning level.

It should be noted that because the authorized vessels maneuver close to an installation, it has been

assumed that the installation is not able to take measures to avoid a collision.

The frequency of collision between a shuttle tanker and an installation, or storage unit, is estimated to be0.0046/year due to failure of the dynamic positioning system [Ref. 1]. It is assumed that 20 percent (i.e.,0.0009/year) of shuttle tanker collisions occur after loading operations are complete and the fully loadedvessel is leaving the field. This relatively low percentage is due to the fact that the shuttle tanker isholding and maintaining position, in order to achieve loading, and is aware of the installations location.In addition, it is usual practice to perform shuttle tanker loading operations at a safe distance from thefacility. The remaining 80 percent (i.e., 0.0037/year) of shuttle tanker collisions are assumed to occur

while the tanker is empty and on approach to the facility.

The failure of the dynamic positioning system on a maintenance support vessel, causing a collision, isestimated to be 0.0137/year [Ref. 1].


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In the event of a collision, the severity of damage that an installation experiences differs, depending onthe impact energy of the collision. The following equation is used to determine vessel collision impactenergy [Ref. 2]:

2)1000/(2/1 kV M E =

Where:E = impact energy (MJ)M = vessel mass (tonnes)V = vessel speed (m/s) = 0.514 x (speed in knots)k = hydrodynamic added mass constant

= 1.1 for head-on (powered) impact= 1.4 for broadside (drifting) impact

Note: It is assumed that for supply/standby vessel collisions, k is equal to 1.4, and for shuttle tanker collisions, k is equal to 1.1.

It is presumed that for supply/standby vessel collisions, the vessel is at maximum mass (i.e., TotalDisplacement = Deadweight Tonnage + Light Ship Weight) since collisions will be more likely duringapproach to the installation. Table 1 illustrates The mass of the vessels that service current Grand Banks

production facilities, which are used as the basis for this assessment, are provided in Table 1.1-1.

Table 1.1-1 Vessel Displacement

Vessel 1 Light Ship Weight(t)

Deadweight Tonnage(t)

Total Displacement 2

(t)Maersk Bonavista/Placentia

2,500 1,800 4,300

Maersk Norseman/Nascopie

4,654 2,088.2 6,742.2

MCM Kometik 27,094.5 126,646.6 153,741.1 Maersk data is from Reference 3 and Kometik data is from Reference 4. All of the Maersk vessels serve as supply and standby duties. Therefore, in this conservative analysis, the TotalDisplacement of Maersk vessels is 6,745 t.

It is conservatively assumed that vessel collisions occur during maneuvering. The percentage of incidents, at various speeds, for historically recorded occurrences is presented in Table 1.1-2 [Ref. 2].


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Table 1.1-2 Maneuvering Collision Mean Speed and Percentage of Incidents

Speed Range(kts)

Mean Speed(m/s)

% of Incidents

0 1 0.3 271 2 0.8 26

2 3 1.3 133 4 1.8 74 5 2.3 205 6 2.8 7

The following categories of impact energy are chosen on the basis of potential damage to theinstallation:

0 to 30 MJ: minor damage to facility 30 to 100 MJ: moderate damage to facility 100 to 200 MJ: heavy damage to facility 200 MJ: catastrophic loss of facility

Impact frequencies have been estimated for each category to allow the event tree modelling to representthe different consequence levels.

Design rules typically require all offshore installations to be capable of withstanding at least 15MJimpacts, however it is likely that actual capacity will exceed this value by a significant amount. The risk assessment assumes that impacts of energy less than 30MJ will not cause damage to the hull, hence thechoice of energy bands in this analysis.

The calculated impact energy, and percentage of incidents are illustrated in Figures 1.1-1 to 1.1-3.


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Figure 1.1-1 Probability Distribution of Impact Energy for Supply/Standby Vessels

Figure 1.1-2 Probability Distribution of Impact Energy for Shuttle Tanker (Full)

0

20

40

60

80100

0 100 200 300 400 500 600 700

Impact Energy (MJ)

C u m m u

l a t i v e

% o

f I n c i

d e n

t s

0

20

40

60

80

100

0 5 10 15 20 25 30 35 40Impact Energy (MJ)

C u m m u

l a t i v e

% o

f I n c i

d e n

t s


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Figure 1.1-3 Probability Distribution of Impact Energy for Shuttle Tanker (Empty)

The resulting IIFs from the above approach are presented in the Fault Trees illustrated in Figures 1.1-4to 1.1-6.

0

20

40

60

80

100

0 20 40 60 80 100 120 140

Impact Energy (MJ)

C u m m u

l a t i v e

% o

f I n c i d e

n t s


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Figure 1.1-4 Fault Tree to Estimate Frequency of Collisions by Authorized Vessels (FPSO)

Authorized vesselcollision with

FPSO

Maintenancesupport vessel

loss of position

0.0046/yr

0.0137/yr

Impact Energy IIF0 - 30 MJ 0.016/yr 30 - 100 MJ 0.00183/yr >100 MJ 0.000477/yr

Total : 0.0183/yr

Shuttle tanker loss of position

Shuttle tanker loss of

position whileempty

Shuttle tanker loss of

position whilefull

0.0037/yr 0.0009/yr


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Figure 1.1-5 Fault Tree to Estimate Frequency of Collisions by Authorized Vessels (Semi-submersible)

0.00056/yr

>100 MJTotal: 0.0137/yr

Authorized vessel collision with

Semi-Sub

Maintenancesupport vessel

loss of position

0.0137/yr

Impact Energy IIF0 - 30 MJ 0.0130/yr 30 - 100 MJ


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In considering a passing vessel collision, the vessel has to be on a collision course with the installation.Due to a lack of historical shipping lane information and traffic data for non-oil related vessels on theGrand Banks in relation to oil and gas operations, passing vessel frequencies can only be estimatedapproximately. A value of 0.00038/year has been recorded for the frequency of passing vesselcollisions with fixed installations [Ref. 5]. This value is based on worldwide data, collected over 89,000 installation years, and is conservatively assumed for the White Rose analysis.

One of the main advantages of a floating installation, as opposed to a fixed facility, is the ability to moveoff-station, as a precautionary measure, in the event that an approaching vessel poses a threat of collision. Riser and mooring systems are designed for controlled and emergency releases. Thecontrolled release includes measures to depressurize risers prior to disconnecting, whereas an emergencyrelease may not.

The disconnection ability for the FPSO will be a highly reliable system, with extensive design effortdevoted to ensuring a high level of availability-on-demand. It is assumed, therefore, that for the FPSO,

FSU and Semi-Sub, the overall probability of disconnection failure is equal to 1 percent.

Once disconnected the facility must also move out of the path of any approaching ship (or iceberg) andthis requires the availability of the thrusters (and power to the thrusters). There will be multiplethrusters, and partial manoeuvrability will be possibility even with only one or two thrusters available.However, even if the facility loses all ability to move under its own power there will still be the optionof a support vessel towing the facility clear of any errant ship or iceberg. It can be concluded thereforethat provided the facility can disconnect then it will be able to avoid a collision.

As with the analysis for authorized vessels, it is clear that the severity of damage that an installationwould experience differs, depending on the impact energy of the collision. Passing vessels are generallylarger than authorized vessels, with the possible exception of shuttle tankers, and in the case of poweredimpacts would generally be travelling at much higher speeds during impact. Sample vessel fleet dataobtained from Maersk and Oceanex are illustrated in Table 1.2-1.

Table 1.2-1 Sample Vessel Data

Company 1 Vessel Type Deadweight Tonnage (DWT)

Maersk Crude Carrier 308,300Maersk Crude Carrier 299,700Maersk Crude Carrier 277,000Maersk Bulk Carrier 68,166Oceanex Container ship 21,849Oceanex Container ship 10,919Oceanex Container ship 14,597 Maersk data are from Reference 6 and Oceanex data are from Reference 7.


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Considering a powered collision between the smallest of the above vessels (i.e., 10,919 DWT), at aspeed of 12 kts (this particular vessel is listed with a speed of 18 kts), the calculated impact energy isgreater than 200 MJ. Consequently, powered passing vessel collisions will conservatively be assumedto result in catastrophic failure of the installation.

It should be noted that a drifting vessel collision would cause considerably less damage than a poweredcollision. The Terra Nova Concept Safety Analysis [Ref. 1] demonstrates that drifting vessels contribute10 percent of the frequency for all passing vessel collisions. Therefore, it is reasonable to assume asimilar proportion for White Rose. Because this is such a small portion of the overall frequency, it is not

justified to investigate this issue any further. Consequently, it is reasonable, but conservative, to assumethat all passing vessel collisions (i.e., both powered and drifting) result in the loss of the installation.

The results of the above approach are presented in the Fault Tree illustrated in Figure 1.2-1.


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Figure 1.2-1 Fault Tree to Estimate Frequency of Collisions by Passing Vessels

3.8 x 10 -6 /yr for FPSO, Semi-sub or FSU

Passing vessel collisionwith facility

Failure torelease

mooring andriser systems

0.01 for FPSO, Semi-sub or FSU 3.8 x 10 /yr -4

Passingvessel oncollision

course withinstallation


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2 REFERENCES

1. Terra Nova Concept Safety Analysis, Magellan Engineering Consultants, 1996

2. A Guide to Quantitative Risk Assessment for Offshore Installations, CMPT Publication 99/100,1999.

3. Personal Communication between David Randell and Captain Bill Morgan (Maersk), March 15 th ,2000.

4. MCM Kometik company brochure.

5. Quantitative Risk Assessment Datasheet Directory, E&P Forum Report No. 11.8/250, 1996.

6. Maersk Website: www.mearsk.com

7. Oceanex Website: www.oceanex.com

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